The following information explains each stage of the treatment process.

1. Coagulation:

The raw water is brought from the intake screens with underground low lift pumps to the coagulant tanks, where liquid alum, aluminium sulphate, Al2(SO4)3.14H2O - a metallic salt, is injected into the tanks to remove particulate impurities from the water, such as non-settleable solids, through adjustment of the solubility limit of the metal hydroxides.

The alum begins a series of reactions with other ions, which form a precipitate commonly called floc. The floc allows particulates such as clay, silt and simple organic structures to absorb onto the surface.

2. Flocculation:

After the floc has begun absorption, a non-ionic synthetic organic polymer is injected into the water to form interparticle bridges that collect and trap the particles. The pH of the water determines the chemical compounds that predominate. Lower pH values favour positively charged species, which are desirable as they react with negatively charged colloids and particulates, and also encourage the formation of insoluble flocs.

Liquid alum favours a pH between 5 and 7. The alkalinity may be modified by the addition of lime or soda ash. This is required if the natural raw water alkalinity is too low, hindering the complete precipitation of alum. After the floc has begun absorption, it passes across a series of baffles and mixed with horizontal paddles.

3. Sedimentation:

The water continues on from the flocculation chambers to the sedimentation tanks, where it is held while the floc, which now consists of quite large particles, about 3mm in diameter, begins to settle out and sink to the bottom of the tank.

As the concentration of solids increases at the base of the tank, the water is pushed up and over the sides of the long narrow weirs that traverse the top of the tanks. This clean filtered water is then sent to the Activated Carbon Treatment bays. The flocculants are sent to nearby sludge storage dams.

4. Activated Carbon Treatment:

Powdered activated carbon is added to the clarified water during flocculation or as the water passes from the sedimentation tank into the filters. This removes taste and odour compounds. It also removes any potential toxins when there is a blue-green algal bloom in the river.

5. Filtration:

Gravity forces the water through filter beds containing sand removing any remaining particles. The trapped particles are periodically removed from the filter by using compressed air and then pumping clean water in the reverse direction. The wastewater flows to sludge collection dams.

6. pH Correction:

After the water has passed through the sand filters, it undergoes a series of analytical tests. If the pH is too acidic, hydrated lime is mixed with the filtered water to bring the pH to just above neutral, so the water is not corrosive to piping.

7. Distribution and disinfection:

The water now flows into two concrete tanks prior to being pumped to reservoirs. The final stage of the water treatment process occurs in these tanks and involves adding gaseous chlorine to the water to kill remaining organisms which could cause diseases.

Process control

Each step of the water supply is computer controlled and monitored 24 hours per day by an operator in the control room of the water treatment plant. The plant uses a computerised control system to monitor all aspects of the process and plants at all times, and will flag any problems for the operating staff. In the event of a problem it can isolate and shut down any affected areas.

Qualified staff perform various tests on the water as it is processed. Samples of treated water from various sites are regularly analysed at the Water Laboratory to ensure a safe and pure supply of water for the community.